This invention relates to an apparatus and method for determining the position of a movable element in a data space, and to improvements in techniques for determining the travel time of acoustic energy over an unknown travel distance between an acoustic transmitter and an acoustic receiver.
Graphical digitizers are conventionally used to input graphical coordinate information, or the like, to a companion system. In a graphical digitizer, wave energy is typically passed between a movable element (such as a stylus or cursor) and one or more transducers located at fixed reference locations. The transit time of the wave energy traveling (in either direction) between the movable element and the reference locations is used in determining the position of the movable element, in one, two, or three dimensions, typically in terms of digital coordinates. A type of graphical digitizer manufactured and sold by the assignee hereof, Science Accessories Corporation, measures the transit time of acoustic or sonic energy propagating through air. The transit time of sound traveling from a source on the movable element to each of a plurality of microphones is used, in conjunction with the velocity of sound in air and known geometrical relationships, to compute the position of the movable element.
The accurate determination of the transit time of the acoustic energy between the transmitter and receiver locations is critical to an accurate determination of the position of the movable element. Typically, a timer is provided for each receiver. All of the timers are started when the acoustic energy is transmitted from the transmitter. As the sound is received at each receiver, the timer associated with that receiver is stopped. The transit times to each receiver can then be computed from the time that elapsed on each timer. Typically, each timer is a digital counter which counts pulses from a digital clock generator, and the arrival of acoustic wave energy at each microphone is determined by continuously comparing the microphone output (e.g. an amplified and filtered version thereof) to a predetermined threshold level. When the threshold level is exceeded, the associated counter is turned off. Ideally, the threshold detection operation is such that it will not be triggered by noise, but will be triggered, in consistent fashion, by the arriving acoustic wave energy from the transmitter. Such ideal result is not always achieved, however.
It is among the objects of the present invention to improve operation of the described type of apparatus.